A giant secret chamber has been discovered in the ancient Egyptian Great Pyramid of Giza. The 100 foot-long room was found using cosmic rays—particles that can be used to penetrate stone and reveal hidden, internal voids without needing to directly access the site.

The Great Pyramid was built during the fourth dynasty between 2580 and 2560 BC, during the reign of pharaoh Khufu, who ruled the ancient civilization for decades. The pyramid, which is 450ft high and 750ft wide, is known as one of the Seven Wonders of the Ancient World—it is estimated to have taken tens of thousands of workers up to 20 years to build.

An aerial view of Khufu’s Pyramid. ScanPyramids mission

ScanPyramids is a project involving an international team of researchers and launched in October 2015 under the authority of the Egyptian Ministry of Antiquities. Its goal was to use particle physics to probe the internal structure of the pyramids in a non-invasive way. The technology works by using muons—popularly known as cosmic rays—to detect anomalies and voids within the pyramids.

Millions of cosmic rays pass through the human body every day and as they do they change trajectory ever so slightly. This means if they were passing through a stone object—the exterior of a giant ancient pyramid for example—and then moved into an empty space, scientists could track and map the changes.

ScanPyramids’ Big Void 3-D Artistic view. ScanPyramids mission

Previously, the ScanPyramids team has said their findings hint at the existence of secret, hidden chambers within the Great Pyramid. The latest findings, published in Nature, are the first to confirm the discovery of a large void within the 4,500-year-old structure.

Researchers say the void they have detected measures around 100 foot in length and 26 foot in height. It sits directly above the Grand Gallery, a 153 foot-long room that connects the chambers of the pyramid, including the Queen’s and King’s chambers. The newly discovered chamber has a similar cross section to the Grand Gallery, indicating that the two are connected.

The discovery could point to a new, undiscovered chamber, or it could be a void that was integral to the construction of the pyramid—our understanding of which poses one of the biggest archaeological mysteries of all time.

In a press conference, the ScanPyramid scientists said there has been no discussion on what the next steps are in terms of drilling into the void, but said emerging technologies now provide new ways to probe these spaces while causing minimal damage to the structure—one option could be a flying robot, they said.

A view inside the Grand Gallery. ScanPyramids mission

To understand the purpose of the void, they said researchers from other disciplines, such as Egyptologists, will be needed to provide context to their discovery.

Kate Spence, Senior Lecturer in Egyptian Archaeology at the University of Cambridge, U.K., who was not involved in the study, tells Newsweek the chamber is more likely to be related to the construction of the pyramid rather than a hidden room full of treasure.

“What they seem to have found is a linear anomaly above a gallery which is a very complex piece of construction in the center of the Great Pyramid,” she says.

“If you look at the cross section of the pyramid, above the kings chamber—the main burial chamber—there are a whole series of little chambers which are roofed with incredibly heavy big granite blocks. My suspicion is what they are looking at is the remains of a constructional ramp which has been built within the pyramid probably for getting those blocks into place.”

She says that what might of happened is that once these huge blocks were in place, the builders did not fill up the space, or it was filled sand or something that less dense than rock—which would account for the results presented. “[The discovery] is very important for thinking that through and providing information on how the pyramid was actually built,” Spence adds.

While the newly-discovered chamber should help experts better understand how the Great Pyramid was built, many mysteries about the ancient monument remain. “One of the problems we have with the Great Pyramid is there are no direct parallels for the internal structure of it. It’s quite unusual, and when it’s unusual and nobody has written anything down about it, it can be very difficult to interpret what’s going on.”

The Grand Gallery, for example, has a sloping base and was used as an approach to the burial chambers. However, it has a “very strange arrangement” as all the passages to get to these chambers are very small—then you get to this huge space you can walk through. “What exactly it was for and why they decided to do that, it is impossible to say at this stage.”

Khufu’s Pyramid 3-D cut aerial view. ScanPyramids mission

The ScanPyramid scientists said the structure of the newly discovered void should now be studied in more detail: “There are still many architectural hypotheses to consider; in particular, the big void could be made of one or several adjacent structures, and it could be inclined or horizontal.

“Overall, this discovery…calls for more interdisciplinary collaborations to help understanding the pyramid and its construction process.”